Scale

Yellowing or wilting leaves signal many different plant problems. But if the degradation is accompanied by small, dome-shaped shells on the leaves, the plant is most likely suffering from a scale infestation. These insects suck the sap and moisture out of plants, killing them in the process. Homeowners can scrape scale off leaves with a table knife or similar object, but if you're dealing with an especially bad case, you’ll need to remove the infected parts of the plant and apply an insecticide.